The epoxides of the above formula (I) are of great interest in the fragrance industry as precursors for the synthesis of ketones of the formula (II), into which they are transformed in a rearrangement reaction which, depending on the conditions chosen, may proceed in a stereochemically selective way. ##STR5##
The ketones of formula (II), which show a (1R)-cis configuration, form a class of fragrant molecules developing a jasmine-like odour which varies depending on the nature of the substituents R and R.sup.1. It has been found that the molecules possessing the configuration (1R)-cis, as shown in the above formula (II), are exactly those responsible for the typical jasmine odour of the compounds, whereas the other 3 stereoisomers contribute to a much lesser extent to the mentioned, and highly appreciated jasmine odour.
Of particular interest for perfumers are the two molecules of formula (II) in which R is a methyl group and R.sup.1 is either a n-pentyl or a (Z)-2-pentenyl group. These compounds represent best the jasmine odour which is so prized by perfumers. In what concerns the compound in which R.sup.1 is a n-pentyl group, this compound has been synthetically available for some time now, for example by enantioselective hydrogenation of appropriate precursor compounds (see U.S. Pat. No. 5,874,600 and WO 98/52687, both to Firmenich S A). Another synthesis for this compound and for its (Z)-2-pentenyl analogue, which is related to the present invention, is described in U.S. Pat. No. 5,962,706 (applicant: Firmenich S A), which will be discussed in greater detail below.
A process for the synthesis of the epoxides of the above-mentioned formula (I) is described in U.S. Pat. No. 5,962,706 (applicant: Firmenich S A). This synthesis comprises the reaction steps which are outlined in the following scheme I. ##STR6##
The above synthesis, although giving high enantiomeric excesses (ee's), of the order of greater than 95%, and high yields and allowing thus to prepare the desired epoxides (I) having the defined stereochemistry, can still be improved in order to raise the overall yield and reduce the costs of the synthesis. In particular, the synthesis of the ester (III), starting from the chiral alcohol (IV), is hampered by the fact that the intermediate products (designed in brackets) which are obtained after the esterification reaction with an orthoester CH.sub.3 C(OR).sub.3 have to be treated at a relatively high temperature of above 140.degree. C. in order to induce the Claisen rearrangement reaction which gives the desired ester (III), from which are prepared the epoxides (I). Therefore, in the case where the esterification of the alcohol (IV) is carried out with trimethylorthoacetate (R.dbd.CH.sub.3), an incomplete rearrangement reaction results because the intermediate ester can only be heated to about 115.degree. C., due to its low boiling point. Thus, use of triethylorthoacetate (R.dbd.C.sub.2 H.sub.5), or even higher analogues is required, in which case the intermediate esters show higher boiling points, permitting a complete conversion in the Claisen rearrangement reaction leading towards the ester (III). However, because the most appreciated compounds of formula (II) are the methyl esters (R.dbd.CH.sub.3), a supplementary transesterification or saponification/esterification reaction has to be carried out after the rearrangement reaction has taken place, thus raising costs and lowering yields.